Saya and her team infiltrate the Yanbaru laboratory. David had learned that the U.S. military is conducting experiments underground to build artificial Chiroptera. There he discovers a mysterious keyword called "Delta 69". Then, a Chiropteran swoops down on Saya's party underground, where they can take no refuge. In order to escape from their subterranean trap, they will have to defeat the creature. Who can save Saya and her party from this desperate predicament?

At the experiment farm of Les Cinq Fleches Pharmaceutical, Saya is surrounded by child Chiroptera. But confronted with a mysterious humming coming from nowhere, the howling of the Chiroptera and a choking smell of blood, the girl goes berserk, and swings the sword at anything that comes into her view without even distinguishing between friend and enemy...